The speeds of metal-oxide-semiconductor (MOS) transistors are closely related to the drive currents of the MOS transistors, which drive currents are further closely related to the mobility of charges. For example, NMOS transistors have high drive currents when the electron mobility in their channel regions is high, while PMOS transistors have high drive currents when the hole mobility in their channel regions is high.
Germanium is a commonly known semiconductor material. The electron mobility and hole mobility of germanium are greater (2.6 times and 4 times, respectively) than that of silicon, which is the most commonly used semiconductor material in the formation of integrated circuits. Hence, germanium is an excellent material for forming integrated circuits. An additional advantageous feature of germanium is that germanium's hole and electron motilities have a greater stress sensitivity than that of silicon. For example, FIG. 1 illustrates the hole mobility of germanium and silicon as a function of uni-axial compressive stresses. It is noted that with the increase in the compressive stress, the hole mobility of germanium increases at a faster rate than silicon, indicating that germanium-based PMOS devices have a greater potential to have high drive currents than silicon-based PMOS devices. Similarly, FIG. 2 illustrates the electron mobility of germanium and silicon as functions of uni-axial tensile stresses. It is noted that with the increase in the tensile stress, the electron mobility of germanium increases at a faster rate than that of silicon, indicating that germanium-based NMOS devices have a greater potential to have high drive currents than silicon-based NMOS devices.
Germanium, however, also suffers from drawbacks. The bandgap of germanium is 0.66 eV, which is smaller than the bandgap of silicon (1.12 eV). This means that the substrate leakage currents of germanium-based MOS devices are high. In addition, the dielectric constant of germanium is 16, and is greater than the dielectric constant of silicon (11.9). Accordingly, the drain-induced barrier lowering (DIBL) of germanium-based MOS devices is also higher than that of silicon-based MOS devices.